Conventionally, spark plugs have been used for ignition in internal combustion engines, such as automobile engines. An ordinary spark plug has a center electrode; an insulator for holding the center electrode in an axial hole thereof; a metallic shell for holding the insulator in a cylindrical hole thereof; and a ground electrode joined to the metallic shell and forming a spark discharge gap in cooperation with the center electrode. The spark plug is mounted to an engine in such a manner as to project the spark discharge gap into a combustion chamber. A spark discharge (initiated through dielectric breakdown of gas and also called an aerial discharge for distinguishing from a creeping discharge to be described later) is generated across the spark discharge gap, thereby igniting an air-fuel mixture.
Meanwhile, the insulator holds the center electrode in a front end portion of the axial hole thereof. The metallic shell holds the insulator such that a holding portion provided in the cylindrical hole thereof is brought in direct or indirect contact with an outer surface of the insulator. The metallic shell and the center electrode are isolated from each other by a portion (hereinafter referred to as the “isolation portion”) of the insulator located frontward of a position where the holding portion of the metallic shell is in direct or indirect contact with the insulator, thereby being insulated from each other.
When high voltage is applied between the metallic shell and the center electrode, which are isolated from each other by the isolation portion, a discharge may be generated on the isolation portion in such a manner that sparks creep on a surface of the insulator; i.e., a so-called creeping discharge may be generated on the isolation portion. When a regular spark discharge gap (i.e., a gap between the center electrode and the ground electrode) is widened due to consumption of the electrode(s) or when the spark discharge gap is intentionally widened in the design of a spark plug for enhancement of ignition performance, a required voltage for initiation of an aerial discharge across the spark discharge gap increases. When voltage to be applied across the spark discharge gap is increased so as to meet the requirement, a creeping discharge may be generated along the isolation portion, potentially resulting in an impairment in the reliability of spark discharge across the regular spark discharge gap.
In order to prevent the occurrence of such a creeping discharge, the axial length of the isolation portion may be increased for increasing insulation distance. However, when insulation distance is increased merely through design to axially elongate the isolation portion, the size of the isolation portion increases, resulting in an increase in thermal capacity. Accordingly, the heat transfer performance of the isolation portion may deteriorate. Then, the spark plug is apt to become a spark plug of a low heat value type (a so-called hot type) and thus may fail to satisfy a heat value requirement of an engine. According to conceivable measures for preventing the problem, for example, the isolation portion is provided with an uneven shape; specifically, corrugations, so as to elongate insulation distance against a creeping discharge along the isolation portion while the axial length of the isolation portion is held unchanged. Through employment of such corrugations, the heat value of the spark plug does not change greatly. Also, even when a required voltage for spark discharge increases, a creeping discharge becomes unlikely to be generated, and an aerial discharge can be generated across the regular spark discharge gap (refer to, for example, Patent Document 1).
As mentioned above, the metallic shell holds the insulator such that the holding portion thereof is brought in contact with an outer surface of the insulator. A gap between the holding portion and the isolation portion as measured at a position located frontward of the contact position is narrower than a gap between the wall surface of the cylindrical hole of the metallic shell and the isolation portion. When a large gap is formed between the holding portion and the isolation portion, upon the occurrence of contamination, the generation of sparks across the gap can be restrained. However, in view of a reduction in the size of a spark plug, the employment of such a large gap is difficult. Under the circumstances, when the gap between the holding portion and the isolation portion was reduced to 0.4 mm or less, entry of unburnt gas into the gap could be prevented, and thus resistance to contamination in the gap could be enhanced; as a result, the generation of sparks across the gap could be prevented (refer to, for example, Patent Document 2).    Patent Document 1: Japanese Utility Model Application Laid-Open (kokai) No. S50-59428    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2002-260817